Accidentally mirrored on an offset, too late to undo
Start with that: Go into edit mode (vertex select) and select all, then in the menue on (most likely) the bottom left of the screen go to mesh --> vertices --> remove doubles and set the merge distance to 0.02
Then in the menue select --> select all by trade --> interior faces and delete them.
That should at least reduce the problem. From here on cleaning the rest by hand should be not too much work. To help with that you can use mesh analyses (in the properties panel).